Update the plugins to use the PluginAPI

Also:
- Add get_conversation_messages() to PluginAPI
- Make plugins_autoload colon-separated instead of space-separated
    (for consistency)
- Replace a JID() with a safeJID() in the uptime plugin
Fix #2254

(add a new theming option, too)
fix the color of the scrolled tab with the dark theme
Fix #2122 (coloration of long messages)

- Add a “prepend” attribute to the Line tuple

I’m not sure of the impact of this on performance (we parse the message
yet another time)